To be used for power modules in electric vehicles (EV), etc., ceramic circuit boards comprising copper sheets bonded to ceramic substrates having good thermal conductivity via brazing materials have become increasingly demanded. Such circuit boards are generally produced by coating substrates of ceramics such as silicon nitride, aluminum nitride, etc. having high thermal conductivity with a brazing material containing an active metal; heat-bonding a metal sheet of copper, aluminum, etc. to the brazing material; printing a resist in a circuit pattern on the metal sheet; and removing unnecessary metal sheet portions covered with no resist by etching.
When a copper sheet is bonded to a sintered silicon nitride substrate with a brazing material comprising Ag, Cu, and Ti (active metal) as described in Japanese Patent 3,629,783, a layer bonding the copper sheet to the ceramic substrate, which is formed by a brazing material containing an active metal, is presumably constituted by an alloy layer comprising Cu and Ag as main components, which is formed on the copper sheet side, and a reaction product layer comprising titanium nitride as a main component, which is formed on the sintered silicon nitride side.
Unnecessary brazing material portions between circuit patterns are removed to some extent when the metal sheet is etched, but an alloy layer (for example, mainly composed of Cu—Ag) and a reaction product layer (for example, mainly composed of titanium nitride) in the brazing material may partially remain without being completely removed as the case may be. A partially remaining brazing material decreases insulation or causing short-circuiting between adjacent circuit patterns.
JP 2006-351988 A discloses the production of a ceramic circuit board by coating both surfaces of a ceramic plate (made of nitrides such as aluminum nitride, silicon nitride, etc.) with a brazing material comprising an active metal, bonding metal sheets to the ceramic plate, and etching a metal sheet with a mask. JP 2006-351988 A describes that because the brazing material, or its alloy layer and nitride layer remain on the ceramic circuit board after removing unnecessary metal sheet portions by etching, they are generally removed with an aqueous ammonium fluoride solution, and an aqueous solution comprising a mineral acid such as sulfuric acid, nitric acid, etc., and hydrogen peroxide. Though an aqueous etching solution containing ammonium fluoride for removing the brazing material can enough remove the reaction product layer mainly composed of titanium nitride, it cannot fully remove the alloy layer, partially leaving a residual brazing material, resulting in decreased insulation or short-circuiting between circuit patterns.
JP 9-162325 A discloses a silicon nitride circuit board comprising metal circuits with or without a heat-dissipating metal plate, each formed on a silicon nitride substrate via a bonding layer containing an active metal, the bonding layer being as thick as 20 μm or less, and the metal circuits and the heat-dissipating metal plate containing 50 ppm or less of oxygen. JP 9-162325 A describes that the silicon nitride circuit board is produced by coating the substrate with a brazing material paste containing an active metal or a compound containing an active metal, bonding metal sheets sufficiently wide to cover the paste, forming circuit patterns on the metal sheet of the bonded body with an etching resist, etching the metal sheet to form metal circuits, and removing the brazing material existing between the metal circuits. JP 9-162325 A describes that to remove the brazing material, a (warm) aqueous solution containing ammonium hydrogen fluoride (NH4F.HF), hydrogen peroxide (H2O2), etc. is used. Though an aqueous etching solution containing ammonium fluoride for removing the brazing material can effectively remove the reaction product layer mainly composed of titanium nitride, it cannot fully remove the alloy layer. Accordingly, the brazing material partially remains, resulting in decreased insulation or short-circuiting between circuit patterns.
To remove unnecessary brazing material portions between circuit patterns completely, it is known that after the copper sheet is etched, the brazing material is further etched. For example, JP 10-154866 A discloses a method comprising a first treatment with an aqueous solution containing ammonium fluoride and hydrogen peroxide, and a second treatment with an aqueous solution containing alkali and hydrogen peroxide. JP 10-154866 A describes that this method can completely remove unnecessary brazing material portions without decreasing the size precision of circuit patterns. However, the brazing-material-removing method described in JP 10-154866 A suffers the erosion of an etching resist with an aqueous alkali solution used.
JP 10-251878 A discloses a method of removing silver or silver chloride appearing by the etching treatment of a copper sheet with an aqueous solution of sodium thiosulfate and/or potassium thiosulfate, and removing residual silver or alloy thereof with an aqueous solution containing NH4F.HF and hydrogen peroxide. JP 10-251878 A describes that an etching resist is not eroded by this method, because an alkaline solution is not used. Though an aqueous solution of sodium thiosulfate and/or potassium thiosulfate can effectively dissolve silver chloride, it does not reduce and dissolve metallic silver and silver alloy. Accordingly, the method of JP 10-251878 A fails to sufficiently remove metallic silver and a silver alloy, so that the brazing material may partially remain, resulting in decreased circuit board quality.
JP 2005-35874 A discloses a method for producing a metal-bonded ceramic substrate by bonding a metal member to at least one surface of a ceramic substrate via a brazing material containing an active metal; coating predetermined surface portions of the metal member with a resist; etching part of the metal member; removing the resist; etching part of the metal layer, which is made of a metal different from the active metal in the brazing material, for example, with an agent containing hydrogen peroxide, ammonia water and EDTA; and selectively etching the active metal layer. JP 2005-35874 A describes as agents for selectively etching the active metal layer, (a) an agent containing carboxylic acid [citric acid, diethylenetriaminepentaacetic acid (DTPA), ethylenediaminetetraacetic acid (EDTA), hydroxyethylethylenediaminetriacetic acid (HEDTA), 1,3-propanediaminetriacetic acid (1,3PDTA), nitrilotriacetic acid (NTA), hydroxyethylidene 2-phosphoric acid (HEDT), etc.], an oxidizing agent, and alkali; or (b) an agent containing a compound having one or more amino groups in its molecule (ethylenediamine compound, etc.), an oxidizing agent, and acid.
However, the production method of a metal-bonded ceramic substrate described in JP 2005-35874 A suffers the corrosion of a metal member surface after removing the resist, because part of a layer made of a metal different from the active metal is removed, and the active metal layer is then selectively etched. When the resist-removing treatment is conducted finally, the method suffers the erosion of an etching resist with an aqueous alkali solution used for removing part of layers of a metal different from the active metal. Also, because an alkaline solution of the agent (a) containing carboxylic acid is used, the method suffers a similar problem. Accordingly, the development of brazing-material-removing technologies carried out in an acidic state has been desired.